Thrombin, phorbol ester, and cAMP regulate thrombin receptor protein and mRNA expression by different pathways

Prognostic value of plasminogen activator inhibitor type 1 mRNA in microdissected glomeruli from transplanted kidneys

BACKGROUND

Plasminogen activator inhibitor type 1 (PAI-1) exerts antifibrinolytic and profibrotic activities. Inside the glomerulus, PAI-1 is mainly synthesized by mesangial cells. We hypothesized that thrombin, via its receptor protease activated receptor type 1 (PAR-1), present on the membrane of glomerular cells, is an important mediator of PAI-1 synthesis.

METHODS

Using the technique of Peten et al., we microdissected the glomeruli of 23 kidney transplanted patients admitted in our department from 1993 to 1997, and we followed-up these patients for up to 5 years, with sometimes iterative renal biopsies. With this technique, we also microdissected the glomeruli of three patients who have had a nephrectomy for cancer (control patients). We investigated mRNA expression of the PAI-1, the thrombin receptor PAR-1, the alpha2 chain of type IV (alpha2 IV) collagen, and of a housekeeping gene (cyclophilin) by reverse transcription-polymerase chain reaction. The results were correlated with the renal function and the histological findings classified into acute rejection (9 biopsies), chronic rejection (22 biopsies), or normal (8 biopsies).

RESULTS

A significant up-regulation of PAI-1 and alpha2 IV collagen mRNA was observed in acute rejection (P<0.05) when compared to normal kidneys. A positive correlation exists between alpha2 IV collagen mRNA level and the degree of cellular infiltration. A negative correlation was found between the level of mRNA of PAR-1 and the degree of vascular thrombosis (P=0.005) and glomerulosclerosis (P=0.04). A positive correlation was found between the degradation of renal function and the mRNA level of PAI-1 at the time of the renal biopsy (P<0.05).

CONCLUSIONS

These results suggest that glomerular PAI-1 mRNA may be predictive of the long-term renal graft function.

Regenerative and proinflammatory effects of thrombin on human proximal tubular cells

Interstitial fibrin deposition is a common histologic feature of tubulointerstitial diseases, which suggests that the coagulation system is activated. Thrombin, generated during the activation of the coagulation cascade, is a powerful activating factor for different cell types. Although proximal tubular cells are potential targets for this coagulation factor, no information is available on the effect of thrombin on these cells. Thus, the expression of protease-activated receptor-1 (PAR-1), the main thrombin receptor, was investigated in human proximal tubular cells (hPTC) in vivo and in vitro. A diffuse expression of PAR-1 was observed by immunohistochemistry along the basolateral membrane of PTC in normal human kidney. This observation was confirmed in vitro in cultured hPTC. Because tubular damage and monocyte infiltration are two hallmarks of tubulointerstitial injury, the effect of thrombin on DNA synthesis and monocyte chemotactic peptide-1 (MCP-1) gene and protein expression was evaluated in cultured hPTC. Thrombin induced a significant and dose-dependent increase in thymidine uptake and a striking upregulation of MCP-1 mRNA expression and protein release into the supernatant. Although PAR-1 is a G protein-coupled receptor, its activation in hPTC, as in other cell systems, resulted in a transient increase in cellular levels of tyrosine-phosphorylated proteins. An increased level of tyrosine-phosphorylated c-src suggested the activation of this cytoplasmic tyrosine kinase in response to thrombin and its potential role in thrombin-induced protein-tyrosine phosphorylation. Interestingly, thrombin-induced DNA synthesis and MCP-1 gene expression were completely blocked by genistein, a specific tyrosine kinase inhibitor, but not by its inactive analogue daidzein, demonstrating a central role for tyrosine kinase activation in the thrombin effects on hPTC. Moreover, the specific src inhibitor PP1 abolished the thrombin effect on DNA synthesis. In conclusion, thrombin might represent a powerful regenerative and proinflammatory stimulus for hPTC in acute and chronic tubulointerstitial diseases.

Tissue factor, plasminogen activator inhibitor-1, and thrombin receptor expression in human crescentic glomerulonephritis

Glomerular fibrin deposition is a common histological feature of crescentic glomerulonephritis (CGN). Tissue factor (TF) is the most powerful activator of the coagulation system, whereas plasminogen activator inhibitor (PAI)-1 is a key modulator of the fibrinolytic pathway. Thrombin, released locally as the final step of the coagulation cascade and trapped within the fibrin clots, can induce the activation of glomerular cells, through the interaction with a specific receptor. To investigate the mechanisms underlying coagulation cascade activation and fibrin deposition and the role of this phenomenon in the pathogenesis of human CGN, TF, PAI-1, and thrombin receptor expression were studied in CGN biopsy specimens. Glomerular TF gene and protein expression were strikingly increased in CGN, in particular within the crescents and in the mesangial area, with the same distribution of fibrin deposits. Interestingly, very few infiltrating mononuclear cells were stained in TF immunohistochemistry. To better evaluate the involvement of monocytes in TF expression, TF mRNA and CD68 protein were studied by an in situ hybridization/immunohistochemistry combined technique. Only 16% of the cells expressing TF mRNA were CD68 positive. However, most of the TF signal was localized in the proximity of monocytes, suggesting that soluble mediator(s) released by these cells could induce TF expression. Indeed, interleukin-1 (IL-1), one of the main monocyte-derived cytokines, upregulated TF mRNA levels in cultured human mesangial cells in a time-dependent manner. Moreover, a striking increase in IL-1 expression was present within the cellular crescents in CGN biopsy specimens. Finally, we observed a marked upregulation of both PAI-1 and thrombin receptor mRNA levels in CGN with a pattern resembling TF and fibrin distribution. Surprisingly, thrombin receptor protein expression was strikingly downregulated in CGN, suggesting its continuous activation and degradation. In conclusion, we can hypothesize that TF and PAI-1, mainly expressed by resident cells, may play a pivotal role in the development and preservation of fibrin deposits in CGN. In addition, thrombin, released locally and accumulated within the fibrin clots, may represent a pathogenetic mediator of crescentic lesions.

Thrombin induces proteinase-activated receptor-1 gene expression in endothelial cells via activation of Gi-linked Ras/mitogen-activated protein kinase pathway

We addressed the mechanisms of restoration of cell surface proteinase-activated receptor-1 (PAR-1) by investigating thrombin-activated signaling pathways involved in PAR-1 re-expression in endothelial cells. Exposure of endothelial cells transfected with PAR-1 promoter-luciferase reporter construct to either thrombin or PAR-1 activating peptide increased the steady-state PAR-1 mRNA and reporter activity, respectively. Pretreatment of reporter-transfected endothelial cells with pertussis toxin or co-expression of a minigene encoding 11-amino acid sequence of COOH-terminal Galphai prevented the thrombin-induced increase in reporter activity. Pertussis toxin treatment also prevented thrombin-induced MAPK phosphorylation, indicating a role of Galphai in activating the downstream MAPK pathway. Expression of constitutively active Galphai2 mutant or Gbeta1gamma2 subunits increased reporter activity 3-4-fold in the absence of thrombin stimulation. Co-expression of dominant negative mutants of either Ras or MEK1 with the reporter construct inhibited the thrombin-induced PAR-1 expression, whereas constitutively active forms of either Ras or MEK1 activated PAR-1 expression in the absence of thrombin stimulation. Expression of dominant negative Src kinase or inhibitors of phosphoinositide 3-kinase also prevented the MAPK activation and PAR-1 expression. We conclude that thrombin-induced activation of PAR-1 mediates PAR-1 expression by signaling through Gi1/2 coupled to Src and phosphoinositide 3-kinase, and thereby activating the downstream Ras/MAPK cascade.

Thrombin: a novel renal growth factor

Glomerular fibrin deposition is a common histopathological finding in a variety of glomerular diseases and suggests the intraglomerular activation of the coagulation system. The activation of thrombin is the last step in the coagulation cascade. On the basis of its ability to modulate several cell functions through the activation of different receptors and intracellular-signaling pathways, thrombin could be considered as a growth factor more than as a simple coagulation factor. Indeed, this multifunctional serine protease may represent the link between the activation of the coagulation cascade and the inflammatory and profibrotic response at the glomerular level. This review will focus on the potential role of thrombin in the pathogenesis of glomerular damage and on the mechanisms underlying the cellular effects of this serine protease.

Differential regulation of protease activated receptor-1 and tissue plasminogen activator expression by shear stress in vascular smooth muscle cells

Recent studies have demonstrated that vascular smooth muscle cells are responsive to changes in their local hemodynamic environment. The effects of shear stress on the expression of human protease activated receptor-1 (PAR-1) and tissue plasminogen activator (tPA) mRNA and protein were investigated in human aortic smooth muscle cells (HASMCs). Under conditions of low shear stress (5 dyn/cm2), PAR-1 mRNA expression was increased transiently at 2 hours compared with stationary control values, whereas at high shear stress (25 dyn/cm2), mRNA expression was decreased (to 29% of stationary control; P<0.05) at all examined time points (2 to 24 hours). mRNA half-life studies showed that this response was not due to increased mRNA instability. tPA mRNA expression was decreased (to 10% of stationary control; P<0.05) by low shear stress after 12 hours of exposure and was increased (to 250% of stationary control; P<0.05) after 24 hours at high shear stress. The same trends in PAR-1 mRNA levels were observed in rat smooth muscle cells, indicating that the effects of shear stress on human PAR-1 were not species-specific. Flow cytometry and ELISA techniques using rat smooth muscle cells and HASMCs, respectively, provided evidence that shear stress exerted similar effects on cell surface-associated PAR-1 and tPA protein released into the conditioned media. The decrease in PAR-1 mRNA and protein had functional consequences for HASMCs, such as inhibition of [Ca2+] mobilization in response to thrombin stimulation. These data indicate that human PAR-1 and tPA gene expression are regulated differentially by shear stress, in a pattern consistent with their putative roles in several arterial vascular pathologies.

Thrombin-induced DNA synthesis of cultured human dental pulp cells is dependent on its proteolytic activity and modulated by prostaglandin E2

To clarify the roles of alpha-thrombin and prostaglandin E2 (PGE2) in the healing and inflammatory processes of dental pulp, their effects on the DNA synthesis of human pulp cells were investigated by measurement of [3H]thymidine incorporation. At a concentration range of 1 to 25 units/ml, alpha-thrombin stimulated DNA synthesis of the pulp cells by 1.5 to 2.6-fold. On the contrary, PGE2 (> 0.05 microgram/ml) suppressed DNA synthesis by 24 to 39%. Using reverse transcriptase-polymerase chain reaction, thrombin receptor mRNA expression was identified in the pulp cells. Furthermore, alpha-thrombin-induced DNA synthesis could be inhibited by antithrombin III (2 units/ml) with heparin (2 units/ml) or D-Phe-Pro-ArgCH2Cl (50 micrograms/ml). PGE2 (0.1 to 0.5 microgram/ml) also inhibited the thrombin-induced DNA synthesis by 39 to 64%. These results imply that pulp cells express the thrombin receptor that is activated by the serine protease activity of thrombin. Interactions of thrombin and PGE2 are important in modulating the inflammatory and healing processes of the pulp.

Proteinase-activated receptors: novel mechanisms of signaling by serine proteases

Although serine proteases are usually considered to act principally as degradative enzymes, certain proteases are signaling molecules that specifically regulate cells by cleaving and triggering members of a new family of proteinase-activated receptors (PARs). There are three members of this family, PAR-1 and PAR-3, which are receptors for thrombin, and PAR-2, a receptor for trypsin and mast cell tryptase. Proteases cleave within the extracellular NH2-terminus of their receptors to expose a new NH2-terminus. Specific residues within this tethered ligand domain interact with extracellular domains of the cleaved receptor, resulting in activation. In common with many G protein-coupled receptors, PARs couple to multiple G proteins and thereby activate many parallel mechanisms of signal transduction. PARs are expressed in multiple tissues by a wide variety of cells, where they are involved in several pathophysiological processes, including growth and development, mitogenesis, and inflammation. Because the cleaved receptor is physically coupled to its agonist, efficient mechanisms exist to terminate signaling and prevent uncontrolled stimulation. These include cleavage of the tethered ligand, receptor phosphorylation and uncoupling from G proteins, and endocytosis and lysosomal degradation of activated receptors.

Serine protease activity is essential for thrombin-induced protein synthesis in cultured human dental pulp cells: modulation roles of prostaglandin E2

Irritations and injuries to the dental pulp usually lead to different degrees of pulpal inflammation. To investigate the roles of thrombin and prostaglandins in the healing and inflammatory processes of dental pulp as well as their effects on pulpal protein synthesis, human dental pulp cell cultures were established and their protein production was measured with or without the presence of exogenous thrombin and prostaglandins. At concentrations of 1-25 U/ml, alpha-thrombin increased the protein synthesis to 1.4-2.3 fold over the vehicle control. On the contrary, 0.1 microg/ml of prostaglandin E1 (PGE1) suppressed protein synthesis by 60%. Prostaglandin E2 (PGE2) also inhibited protein synthesis with an IC50 of 0.4 microg/ml. The stimulatory effects of thrombin (10 U/ml) can be inhibited by antithrombin III (2 U/ml) (a natural thrombin inhibitor) with heparin (2 U/ml), PPACK (D-Phe-Pro-ArgCH2Cl) (20-50 microg/ml) (a serine protease inhibitor), and PGE2 (0.5-1.0 microg/ml). Moreover, TRAP (20-40 microg/ml), a thrombin receptor agonist peptide, also exerted a stimulatory effect (1.21-1.37 fold). In conclusion, thrombin-induced protein synthesis by pulp cells is dependent on proteolytic activity, but not on binding to receptors. Both PGE1 and PGE2 exert suppressive effects on protein synthesis, indicating that interactions between thrombin and prostaglandins are important in regulating the inflammation, repair and regeneration of pulp tissue following injury.

Thrombin receptor-mediated increase of two matrix metalloproteinases, MMP-1 and MMP-3, in human endothelial cells

Matrix metalloproteinases (MMPs) are responsible for the degradation of extracellular matrix components and are secreted by a variety of cells including human endothelial cells. Because alpha-thrombin is known to interact with matrix components and has been shown to activate latent MMP-2 in human umbilical vein endothelial cells, we investigated whether human alpha-thrombin could also regulate other MMPs secreted by the human saphenous vein or mammary artery endothelial cells (EC). After treatment of EC with increasing concentrations of thrombin for different periods of time, a significantly higher gelatinolytic activity of both MMP-1 and MMP-3 was observed in addition to MMP-2 activation. The effect of thrombin was time and dose-dependent, reaching a maximum at 24 hours. After treatment with 5 NIH U/ml thrombin for 24 hours, Western blotting revealed 9.5- and 4.4-fold increases over control values for MMP-3 and MMP-1, respectively. The synthetic thrombin receptor agonist peptide SFLLRNPNDKYEPF fully reproduced the action of thrombin, whereas chemical inactivation of the catalytic site of thrombin abolished its effect on MMP-1 and MMP-3. Thrombin and SFLLRNPNDKYEPF both induced MMP-3 mRNA synthesis but had no significant influence on constitutive MMP-1 mRNA levels. These results demonstrate that thrombin not only activates latent MMP-2 but also modulates MMP-1 and MMP-3 production in EC, this latter effect being mediated by the G-protein-coupled thrombin receptor. Hence, our present data provide evidence to support the suspected role of thrombin in tissue remodeling and angiogenesis.

Thrombin receptor on rat primary hippocampal neurons: coupled calcium and cAMP responses

We have tested the hypothesis that hippocampal neurons respond to thrombin via a neuronal thrombin receptor. A human neuroblastoma cell line, SK-N-SH, known to be thrombin responsive morphologically, responded both to thrombin and thrombin receptor agonist peptide (TRAP 42-55) with elevation of intracellular calcium. In Western blots of membranes from SK-N-SH cells and cultured rat hippocampal neurons using an antibody against the N-terminal peptide of the human thrombin receptor, putative receptor proteins of 66 and 47 kDa were detected in both cells. Neurons were treated with thrombin and TRAP 42-55 (TRAP-14) to determine their effects on intracellular levels of calcium and cAMP. Only 10% of the neurons showed a rapid response to thrombin, but most responded rapidly to agonist peptide with a prolonged elevation of intracellular free calcium. Neuronal cAMP levels were decreased by 40% after 24 h thrombin treatment. This decrease in cAMP level could be blocked by both the Gi-protein inhibitor, pertussis toxin, and the thrombin inhibitor, hirudin, suggesting a possible involvement of Gi-protein-coupled receptor activation. Furthermore, rapid calcium and cAMP responses were apparently induced by pre-treatment of neurons with thrombin for 24 h and subsequent washout. In summary, these data indicate that rat primary hippocampal neurons have thrombin receptors whose responses to thrombin apparently are up-regulated by 24 h thrombin pre-treatment. These results may have implications for synaptic remodeling, learning and memory.

Down-regulation of P2U-purinergic nucleotide receptor messenger RNA expression during in vitro differentiation of human myeloid leukocytes by phorbol esters or inflammatory activators

HL-60 human promyelocytic leukocytes express G protein-coupled P2U-purinergic nucleotide receptors (P2UR or P2Y2R) that activate inositol phospholipid hydrolysis and Ca24 mobilization in response to ATP or UTP. We examined the expression of functional P2UR and P2UR mRNA levels during in vitro differentiation of HL-60 cells by dibutyryl-cAMP (Bt2cAMP), which induces a granulocyte/neutrophil phenotype, or by phorbol-12-myristate-13-acetate (PMA), which induces a monocyte/macrophage phenotype. Both P2UR function and P2UR mRNA levels were only modestly attenuated during granulocytic differentiation by Bt2cAMP. In contrast, P2UR function, as assayed by either Ca2+ mobilization or inositol trisphosphate generation, was greatly reduced in PMA-differentiated cells. This inhibition of P2UR function was strongly correlated with PMA-induced decreases in P2UR mRNA levels, as assayed by Northern blot analysis or reverse transcription-polymerase chain reaction-based quantification. Although PMA induced an early, transient up-regulation of P2UR mRNA, this was rapidly followed by a sustained decrease in P2UR mRNA to a level 5-10-fold lower than that in undifferentiated HL-60 cells. The half-life of the P2UR transcript in HL-60 cells was approximately 60 min, and this was not affected by acute exposure (< or = 4 hr) to Bt2cAMP or PMA. PMA down-regulated P2UR mRNA in THP-1 monocytes and HL-60 granulocytes but not in A431 human epithelial cells or human keratinocytes. P2UR mRNA was also down-regulated in THP-1 monocytes differentiated into inflammatory macrophages by gamma-interferon and endotoxin. These data indicate that myeloid leukocytes possess tissue-specific mechanisms for the rapid modulation of P2UR expression and function during differentiation and inflammatory activation.

The proteinase-activated receptor 2 is induced by inflammatory mediators in human endothelial cells. Comparison with the thrombin receptor

The proteinase-activated receptor 2 (PAR-2) belongs to the family of seven transmembrane region receptors, and, like the related thrombin receptor, it is activated by specific proteolytic cleavage of its extracellular amino terminus. It is not known which proteinase is the physiological activator of the PAR-2, but candidates can be found among the enzymes involved in the inflammatory cascade systems. Here, we have studied the effects of various mediators on the expression of the PAR-2 and the thrombin receptor in cultured human umbilical vein endothelial cells. Stimulation with the cytokines tumor necrosis factor alpha or interleukin-1 alpha as well as bacterial lipopolysaccharide elevated the expression of PAR-2 in a dose-dependent manner. The time course of induction after cytokine stimulation was similar to those published for the adhesion molecules intercellular adhesion molecule-1 and vascular cell adhesion molecule-1. After 20 h of stimulation, PAR-2 mRNA and protein levels were increased to 5-10-fold basal values, and, in the continued presence of tumor necrosis factor alpha, PAR-2 mRNA expression was found to remain elevated for up to 4 days. In contrast, the thrombin receptor gene was not induced by any of these inflammatory mediators. The responses to phorbol ester treatment also differed between the two genes. Thrombin receptor mRNA levels decreased steadily up to 20 h, whereas PAR-2 mRNA levels first rose to about 3-fold basal values at 4 h before decreasing again. Cell surface protein levels of both receptors were decreased after 20 h of phorbol ester stimulation. Elevating intracellular cAMP levels by treatment with forskolin resulted in decreased expression of both receptors, and inhibition of cAMP degradation appeared to blunt the cytokine-induced increase in PAR-2 expression. The induction of the PAR-2 by cytokine treatment supports the concept of PAR-2 involvement in the acute inflammatory response.

Thrombin receptor actions in neonatal rat ventricular myocytes

Previous studies established that thrombin stimulates phosphoinositide hydrolysis and modulates contractile function in neonatal rat ventricular myocytes. The present study further defines the signaling pathways activated by the thrombin receptor and their role in thrombin's actions in cardiac myocytes. The thrombin receptor-derived agonist peptide (TRAP, a portion of the tethered ligand created by thrombin's proteolytic activity) stimulates the rapid and transient accumulation of inositol bis- and tris-phosphates (IP2 and IP3, respectively), which is followed by the more gradual and sustained accumulation of inositol monophosphate (IP1). TRAP elicits a larger and more sustained accumulation of IP1 than does thrombin. Thrombin and TRAP also activate mitogen-activated protein kinase (MAPK) in cultured neonatal rat ventricular myocytes. Differences in the kinetics and magnitude of thrombin- and TRAP-dependent inositol phosphate (IP) accumulation are paralleled by differences in the kinetics and magnitude of thrombin- and TRAP-dependent activation of MAPK. Pretreatment with phorbol 12-myristate 13-acetate (PMA) to downregulate protein kinase C (PKC) attenuates thrombin- and TRAP-dependent activation of MAPK, although small and equivalent effects of thrombin and TRAP to stimulate MAPK persist in PMA-pretreated cells. These results support the notion that the thrombin receptor activates MAPK through PKC-dependent pathways and that the incremental activation of MAPK by TRAP over that induced by thrombin is the consequence of enhanced activation through the PKC limb of the phosphoinositide lipid pathway. TRAP also increases the beating rate of spontaneously contracting ventricular myocytes and elevates cytosolic calcium in myocytes electrically driven at a constant basic cycle length. The effects of TRAP to modulate contractile function and elevate intracellular calcium are not inhibited by tricyclodecan-9-yl-xanthogenate (D609, to block TRAP-dependent IP accumulation) or pretreatment with PMA (to downregulate PKC). The TRAP-dependent rise in intracellular calcium also is not inhibited by verapamil or removal of extracellular calcium but is markedly attenuated by depletion of sarcoplasmic reticular calcium stores by caffeine. Patch-clamp experiments demonstrate that TRAP elevates intracellular calcium in cells held at a membrane potential of -70 mV. Taken together, these results support the conclusion that the thrombin receptor modulates contractile function by mobilizing intracellular calcium through an IP3-independent mechanism and that this response does not require activation of voltage-gated ion channels.

The proteinase activated receptor-2 (PAR-2) mediates mitogenic responses in human vascular endothelial cells

Proteolytically cleaved receptors, typified by the functional thrombin receptor (TR), represent a novel class of receptors that mediate signaling events by functional coupling to G proteins. Northern blot analysis completed with a human proteinase activated receptor-2 (PAR-2) cDNA as probe demonstrated the approximately 3.5kb PAR-2 transcript in total cellular RNA from human umbilical vein endothelial cells (HUVEC). Microspectrofluorimetry using Fura2-loaded HUVEC demonstrated a dose-dependent elevation in intracellular calcium transients ([Ca2+]i) to murine PAR39-44 (SLIGRL, putative neoligand after cleavage), with an approximate EC50 of 30 microM, and evidence for homologous desensitization with complete recovery at 45 min. Xenopus oocytes microinjected with TR cRNA failed to respond to 200 microM PAR39-44, and TR-targeted antisense oligonucleotides specifically abrogated thrombin-induced but not PAR39-44-mediated [Ca2+]i, excluding the possibility that TR/PAR-2 cell-surface coexpression was structurally linked. HUVEC incubated with PAR39-44 demonstrated a dose- and time-dependent mitogenic response similar to that seen with thrombin or TR42-47 (TR-activating peptide, SFLLRN). Preactivation of HUVEC with either PAR39-44 or thrombin resulted in heterologous desensitization to the corresponding agonist, an effect that was mediated primarily by TR internalization as evaluated by immunofluorescence and quantitative ELISA. These results ascribe a previously unrecognized function to the PAR-2 receptor, imply that a natural enzyme agonist may circulate in plasma, and suggest the presence of an additional regulatory mechanism controlling receptor activation events in vascular endothelial cells.